Tomographic volume imaging techniques provide enhanced information over conventional two-dimensional mammography and can help to provide data that lead to early diagnosis and treatment of breast cancer. Cone beam computed tomography (CBCT), for example, is acknowledged to have particular advantages for providing volume images that can be suitable for 3-D mammography. The volume images that can be obtained using tomosynthesis, CBCT, and related techniques have the potential to provide information that can be otherwise difficult to ascertain using conventional two-dimensional (2-D) image modalities.
Volume imaging of a subject requires coordinated movement of the radiation source and sensor, with one or both moving over at least a partial revolution about the subject, capturing a succession of two-dimensional (2-D) images at various angular increments. Data from the individual 2-D images is then used to reconstruct a three-dimensional (3-D) or volume image. The angular range within which a subject is scanned, from 0 to a full 360 degrees, affects the amount of 3-D information that can be reconstructed in a volume image. At the one extreme, an angular range of 0 degrees simply corresponds to a standard 2D projection image with no 3-D information. At the other extreme, an angular range of the full 360 degrees corresponds to a typical CBCT exam with complete 3-D information. Tomosynthesis is typically done at angular ranges that are less than 90 degrees and that allow for 3-D reconstruction with some amount of volume information, sometimes termed “quasi-3-D image reconstruction”. To reconstruct volume images with full 3-D detail, an angular range of at least 180 degrees plus the fan angle of the radiation source used in the system is required.
For breast imaging, the task of acquiring multiple projections from different angles is complicated by the human anatomy itself, which makes it difficult to position the patient in order to obtain the desired field of view for each of the succession of images that are needed. Optimal imaging conditions would be obtained, for example, by positioning the breast so that the orbit of the source and sensor offers the best possible field of view over all angles being imaged. Attempts to achieve this goal, however, are constrained because of the relative anatomical position of the breast and the limits within which the breast tissue can be extended without considerable patient discomfort.
Proposed solutions for maximizing field of view in breast volume imaging tend to compromise either angular range or patient comfort to achieve this goal. A number of system solutions, for example, take two tomosynthesis scans (cranio-caudal CC and medio-lateral oblique MLO), each within a limited angular range of around ±15 degrees, to provide quasi 3-D information for each scan. Other solutions acquire one scan over a 360 degree range for a full 3-D reconstruction and require the patient to be in a prone position, with the breast pendant through an orifice, using gravity to maximize breast extension into the object field of the volume imaging apparatus. For such a system, however, the patient may be required to bend her back or neck in an awkward and potentially painful attitude during the imaging session. Still other solutions propose using suction or other means to pull forward, into the imaging area, as much of the breast tissue as possible. Such solutions can also lead to patient discomfort.
A workable volume imaging apparatus for 3-D mammography would address each of the following objectives: (i) maintain sufficient field of view to increase, by as much as possible, the amount of breast tissue that can be imaged; (ii) allow comfortable patient positioning, so that the head and neck can be naturally supported during imaging; (iii) allow readily adjustable mechanisms for accommodating the patient, with suitable technician access and features for proper breast positioning and variable extension into the imaging area as needed; and (iv) provide a range of imaging modalities, such as conventional mammography imaging, tomosynthesis, and full 3-D imaging such as CBCT or wide angle tomosynthesis with an angle >=180 degrees plus the fan angle from a single system.
Conventional solutions for volume imaging of the breast have fallen short of some of these objectives and typically compromise one or more of imaging quality, patient comfort, field of view, range of modalities, cost, and ease of adjustment.